Abstract

Background

Invertebrate biominerals are characterized by their extraordinary functionality and
physical properties, such as strength, stiffness and toughness that by far exceed
those of the pure mineral component of such composites. This is attributed to the
organic matrix, secreted by specialized cells, which pervades and envelops the mineral
crystals. Despite the obvious importance of the protein fraction of the organic matrix,
only few in-depth proteomic studies have been performed due to the lack of comprehensive
protein sequence databases. The recent public release of the gastropod Lottia gigantea genome sequence and the associated protein sequence database provides for the first
time the opportunity to do a state-of-the-art proteomic in-depth analysis of the organic
matrix of a mollusc shell.

Results

Using three different sodium hypochlorite washing protocols before shell demineralization,
a total of 569 proteins were identified in Lottia gigantea shell matrix. Of these, 311 were assembled in a consensus proteome comprising identifications
contained in all proteomes irrespective of shell cleaning procedure. Some of these
proteins were similar in amino acid sequence, amino acid composition, or domain structure
to proteins identified previously in different bivalve or gastropod shells, such as
BMSP, dermatopontin, nacrein, perlustrin, perlucin, or Pif. In addition there were
dozens of previously uncharacterized proteins, many containing repeated short linear
motifs or homorepeats. Such proteins may play a role in shell matrix construction
or control of mineralization processes.

Conclusions

The organic matrix of Lottia gigantea shells is a complex mixture of proteins comprising possible homologs of some previously
characterized mollusc shell proteins, but also many novel proteins with a possible
function in biomineralization as framework building blocks or as regulatory components.
We hope that this data set, the most comprehensive available at present, will provide
a platform for the further exploration of biomineralization processes in molluscs.